Debug traces show that in per-bdi writeback, the inode under writeback almost always get redirtied by a busy dirtier. We used to call redirty_tail() in this case, which could delay inode for up to 30s.

This is unacceptable because it now happens so frequently for plain cp/dd, that the accumulated delays could make writeback of big files very slow.

So let's distinguish between data redirty and metadata only redirty. The first one is caused by a busy dirtier, while the latter one could happen in XFS, NFS, etc. when they are doing delalloc or updating isize.

Commit b3af9468ae only does that for kupdate case because requeue_io() wasonly called in the kupdate case. Now we are merging the kupdate and !kupdatecases in patch 6/6 (why not?), so is this patch.

> The patch is... surprisingly complicated, although the end result> looks OK. This is not aided by the partial duplication between> mapping_tagged(PAGECACHE_TAG_DIRTY) and I_DIRTY_PAGES. I don't think> we can easily remove I_DIRTY_PAGES because it's used for the> did-someone-just-dirty-a-page test here.

I double checked I_DIRTY_PAGES. The main difference to PAGECACHE_TAG_DIRTY is:I_DIRTY_PAGES (at the line removed by this patch) means there are _new_ pagesget dirtied during writeback, while PAGECACHE_TAG_DIRTY means there are dirtypages. In this sense, if the I_DIRTY_PAGES handling is the same asPAGECACHE_TAG_DIRTY, the code can be merged into PAGECACHE_TAG_DIRTY, as thispatch does.

The other minor differences are

- in *_set_page_dirty*(), PAGECACHE_TAG_DIRTY is set racelessly, while I_DIRTY_PAGES might be set on the inode for a page just truncated. The difference has no real impact on this patch (it's actually slightly better now).

- afs_fsync() always set I_DIRTY_PAGES after calling afs_writepages(). The call was there in the first day (introduce by David Howells). What was the intention, hmm..?

> This code is way too complex and fragile and I fear that anything we do> to it will break something :(